The invention concerns a gradient coil to generate switched magnetic field gradients in a nuclear spin resonance (NMR) device consisting of one or more current paths following windings and being arranged on the surface of a geometric body.
Such a gradient coil is for example known from U.S. Pat. No. 5,323,135.
An essential component of NMR systems, being generally employed for tomography, in part, however, also for spectroscopy, is a system of usually three gradient coils each consisting of several partial coils, which gradient coils are independently of each other supplied with currents of different strengths. These gradient coils serve the purpose to superimpose constant magnetic field gradients with adjustable strengths onto the homogeneous magnetic field B.sub.0 of the NMR device which is directed along a z axis, whereby the direction of one of these gradients (dB.sub.z /dz) is in general parallel to the direction of the homogeneous main field B.sub.0z (z gradient=axial gradient) and the directions of the two other gradients (dB.sub.z /dx, dB.sub.z /dy) are directed orthogonal thereto and to each other and perpendicular to the direction of the main field (x and y gradients=transverse gradients). The spatial region where the magnetic field of these gradient coils varies approximately linearly can be used for volume selective NMR methods (imaging, volume selective spectroscopy) if this region is not further limited by inhomogeneities of the main field.
The gradient coils may for example be realized as x, y and z coils on cylinder surfaces for conventional tomography magnets or as gradient coils for gradient accelerated NMR spectroscopy. Apart from this, planar gradient plates for magnets with pole pieces are also known in NMR tomography. With respect to the geometric design of gradient coils, we refer to the entire contents of commonnly assigned U.S. patent application Ser. No. 08/560,874 and U.S. Pat. No. 5,563,567, where the spatial construction of gradient coils is described in detail.
In the above cited U.S. Pat. No. 5,323,135, a particularly favorable gradient coil system is described, where, for predetermined boundary conditions, on the one hand the inductance L and, apart from this, also further technically relevant parameters of the magnet coil device can be optimized independently of each other, like for example current density distributions, shielding effects, etc.. However, a disadvantage of such a gradient coil device is the effect that for fast gradient switching with such a streamline shaped gradient coil eddy-current-like current distributions on the broad conductor strips are generated distorting the magnetic field of the gradient coil across the volume of interest of the NMR apparatus as a function of time.
It is therefore the purpose of the present invention to improve a gradient coil of the above mentioned kind in such a way that eddy current generation by current redistributions on the conductor strips of the gradient coil and thereby a time-dependent distortion of the magnetic field gradient generated across the volume of interest are considerably reduced.